ELECTRONICS LETTERS 22nd July 1999 Vol. 35 No. 15 Passive temperature compensation of piezo- tunable fibre Bragg gratings A. Richter, T. Andritschke, H. Bock, P. Leisching, D. Stoll, L. Quétel and S. Aguy A new method is presented for the passive temperature compensation of piezo-tunable fibre Bragg gratings using the negative thermal expansion coefficient of the low voltage piezoelectric ceramic. A centre wavelength shift of only 0.5pm/K over a wide range from –10 to 60°C has been shown experimentally. Introduction: Tunable fibre Bragg gratings (FBG) have been studied extensively for filter applications in wavelength division multiplexing (WDM) systems [1 – 6]. They are a very promising tool for the dynamic routing of dense WDM signals in reconfigurable optical add-drop multiplexers (OADMs) [2]. In real world applications, reconfigurable optical transport systems using the 100 or 50GHz ITU frequency-grid require a repeatable tunability and a thermal stability of the FBG centre wavelength in the picometre/K (pm/K) regime. The tunability can be realised by using either thermo-optic [3, 4] or mechanical strain effects [5, 6]. Mechanical strain is applied to an FBG by means of magnetic [5] or piezoelectric [6] forces. Thermal detuning effects impair the FBG wavelength stability. Thermal variations in the FBG wavelength can be controlled actively (temperature control of the FBG environment) or passively. Passive athermalisation techniques are preferable from a system viewpoint because of the high power consumption of active methods and their insensitivity to power failures. An inverse thermal expan- sion is imposed on the FBG by an external mount [7] or by a sub- strate material (glass ceramic) possessing a negative thermal expansion coefficient [8]. However, these conventional passive meth- ods prevent the tunability of the FBG centre wavelength by remote control. In this Letter we report a piezo-tunable, but passively tempera- ture-compensated FBG module for application in reconfigurable OADMs. We exploit the negative thermal expansion coefficient of a low voltage piezoelectric ceramic in order to athermalise the tunable FBG in the operating regime. Experiment: The FBG module investigated in our experiments is depicted schematically in Fig. 1a. The module consists of an optical fibre including a 16mm long fibre Bragg grating. The fibre is glued under mechanical tension to a standard low-voltage piezoelectric stack actuator (piezo). A piezo bias voltage between –10 and 150V leads to an expansion of the piezo between –1 and 15 μm. This expansion causes a change in the FBG centre wavelength of up to 1.6nm by increasing the FBG period. The reflection spectra were obtained by scanning a tunable laser (HP 8168 C, resolution 5pm) and monitoring the reflected power as depicted in Fig. 1b . The tem- perature of the FBG module was controlled from –10 to 60 °C by means of a temperature test chamber. R esults and discussion: F ig. 2 shows the evolution of the reflection spectra of three tunable FBG modules as a function of temperature. Fig. 2a represents the reflection spectra of the FBGs if the piezo of FBG 1 is connected to the voltage supply. The piezo contacts of FBGs 2 and 3 are disconnected. The centre wavelength of FBG 1 reveals a minor temperature dependence of < 1pm/K, whereas the centre wavelength shifts of FBGs 2 and 3 correspond to ~10pm/K. Similar results are observed if the piezo of FBG 2 is connected to the voltage supply (Fig. 2b). Here, the thermal shift in the centre wavelength of FBGs 1 and 3 is 10pm/K, whereas the centre wave- length of FBG 2 is nearly constant (< 1pm/K). The dependence of the centre wavelength on temperature and piezo voltage is given in F ig. 3. The centre wavelength shift of FBG 1 is plotted against tem- perature and piezo voltage. For comparison the temperature depend- encies of a static standard FBG without piezoelectric stack and of FBG 1 with free piezo contacts are shown. The standard FBG exhibits a centre wavelength shift of ~10pm/K. We observe the same temperature variation in the case of disconnected FBG piezo con- tacts. Applying a voltage to the FBG piezo leads to a different behaviour. The average temperature sensitivity is reduced drastically from 10 to 3.2pm/K (0V), 0.5pm/K (20V) and –1.9pm/K (40V), respectively. The minimum shift over a wide temperature range (∆T = 70 K) is achieved at a piezo voltage of ~20V. Fig. 1 Experimental setup a Piezo-tunable fibre grating module b Reflection measurement setup Fig. 2 Reflection spectra of three FBGs at different temperatures a FBG 1: 0V; FBG 2 and 3: piezo contacts disconnected b FBG 2: 0V; FBG 1 and 3: piezo contacts disconnected ——— T = 0 °C – – – – T = 20 °C –·–·–·– T = 40 °C Fig. 3 Centre wavelength shift of FBG 1 and static standard FBG without piezoelectric stack against temperature and piezo bias voltage V  standard FBG (without piezoelectric stack) ·········· gradient: 10.3pm/K FBG 1:  piezo contacts disconnected ◆ 0V, 3.2pm/K ● 20V, 0.5pm/K ▲ 40V, –1.6pm/K